In my mind (in other words, this is arbitrary), books fall into three different categories:

1. Storytelling - all fiction falls under this umbrella, for example.

2. Educational - meant to instruct or teach. Textbooks.

3. Ideas - almost all nonfiction is written with this as its currency.

In my very limited experience as a reader, the most insightful and interesting books are the ones that manage to achieve a blend of all three of these.